The 38-year-old actress, who announced her split from husband Shane Deary after almost seven years of marriage in December, started dating her 'The Americans' co-star earlier this year, and the couple are now inseparable.

A source close to the duo told Us Weekly magazine: ''They try to be discreet but spend all their time together.''

The brunette beauty, who has son River, six, and daughter Willa, two, with her estranged husband, and the 39-year-old British actor are already very serious about each other, but ''are taking it slow.''

The insider added: ''They're in love and know this is for real!''

Speculation first began swirling in January that the former 'Felicity' star was dating Matthew after they were spotted furniture shopping together in Brooklyn Heights, New York City.

Matthew later admitted that he didn't mind being linked to Keri, saying: ''That's the best [rumour] I love.''

The brunette beauty announced her split from building contractor Shane before Christmas but a source previously revealed they separated last summer.